You might be seeing that your keyword rankings fluctuate a lot. Maybe one day you rank in one position, and then the next time it's moved.
First off - don't worry about this movement! It is completely natural, as Google are frequently updating their algorithms.
The rate of fluctuation will vary, dependant upon where the keyword is ranking. It breaks down into two groups:
If your keyword is ranking page 4 or more = then the fluctuations will be a lot more extreme. You can move from page 5 to page 8, back to page 6, and then page 9, etc... This is normal behaviour past page 4 onwards.
If your keyword is ranking page 4 or less = you will see a lot less extreme fluctuations (eg it should not jump 2+ pages back and forth), but you can see it move up-to half a page at a time. Again, this is normal for almost every website.
Hope this helps - give us a shout if you have any questions!